This invention relates to production of metal products, such as billets or slabs, and is particularly directed to a novel process for fusing metals, particularly but not exclusively, reactive metals such as titanium, zirconium, and alloys thereof, and especially in the form of scrap and/or sponge metal, and solidifying such metals under conditions to produce a metal billet or slab which is chemically homogeneous, fine grained and free of freezing segregation
Conventionally, wrought metal products are made by breaking down cast ingots through a variety of hot working processes (forging rolling, extrusion) and/or cold working plus annealing to a desired final size and shape. Economic considerations, primarily minimizing scrap losses due to surface defects and trimming during breakdown, dictate casting ingots as large as possible. Unfortunately the resultant slow solidification leads to chemical segregation and large grain size. Consequently, many of the procedures used for ingot breakdown to final product are aimed at mitigating these deficiencies; however, they are not usually completely successful, and current mill products frequently incorporate chemical inhomogeities together with larger than optimum grain size.
The deficiencies of conventional practices are responsible for a significant portion of product costs, degradation of part fabrication characteristics and lowering of design allowables. The energy and capital equipment costs for breaking down large ingots are substantial. Other disadvantages of conventional practice include the cost of remelting large chunks of mill scrap, the metal lost as oxidized particles during removal of surface defects by grinding, and a second melt employed to improve ingot surface quality and reduce chemical inhomogeneities originating from raw material.
Additionally, part fabrication costs are detrimentally affected by large grain size and segregation. They lead to high rejection scrap rates in forming and forging operations and in magnetic inspection of high strength low alloy and stainless steel parts. Attempts to alleviate these effects, e.g., by reducing the rate of superplastic forming, usually only provide a slightly lower cost solution.
These efficiencies also detrimentally affect performance in service and therefore they inhibit design allowables. Among the properties reduced are strength, ductility, fracture toughness and stress corrosin resistance, and increase in fatigue crack propagation. These disadvantages could be substantially alleviated if current materials were finer grained and freer from segregation.
At present, approximately 25% of all titantium produced as ingots ends up as light fabrication scrap (chips, sheet trimmings, tube trimmings) which is not recycled. This material finds its way into the steel and aluminum industries at a price of a few cents per pound. It is not practical to consolidate such scrap in a non-consumable arc melting furnace because of the concentrated heat source. It is somewhat practical to consolidate it by electron beam melting, but the high vacuum depletes some of the alloying elements which must be replaced in subsequent double consumable melting. It is also somewhat practical to blend about 30% light scrap with sponge and master alloys, and produce conventional electrodes by compacting and welding; these must then be double consumable melted to obtain usable ingots.
Normally, titanium is double melted to homogenize the composition and improve the ingot surface. However, the deep molten pool resulting from the high melting rate used during the second melting for ingot surface improvement leads to segragation during freezing. Also, practicality requires very differnt melting and freezing rates at the bottom, center and top of the ingot. Therefore, inhomogeneity from surface to core and from top to bottom results.
Methods and apparatus for the melting and casting of metals such as titanium, and for scrap metal treatment, e.g. titanium scrap, and their recovery, are illustrated by U.S. Pat. Nos. 3,771,585; 3,417,808; 3,660,074; 3,385,494; and 3,843,352.
One object of the present invention is the provision of a process for melting and then freezing or solidifying a metal, e.g. titanium, to produce a billet or slab which is homogeneous and free of segregation.
Another object is to provide procedure for melting and then freezing or solidifying a metal, e.g. titanium, to produce a billet or slab having fine grain.
Still another object is the provision of a process for melting and then freezing or solidifying a metal, using metal scrap and/or metal sponge, particularly titanium, to produce a billet or slab which is homogeneous and free of freezing segregation, and having fine grain.
Other objects and advantages of the invention will appear hereinafter.